Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (Xixi Campus), Hangzhou 310028, ChinaState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China
r t i c l e i n f o
rticle history:eceived 29 September 2010eceived in revised form 10 February 2011ccepted 16 March 2011vailable online 31 May 2011
a b s t r a c t
Extremely small-size Au nanoparticles mainly distributed at 1–5 nm were successfully prepared onlayered double hydroxide by ion-exchange and reduction procedures (Au/LDH). This catalyst showedsuperior catalytic properties in aerobic oxidation of a wide range of secondary and primary alcoholsunder very mild conditions (e.g. 1 atm pressure of oxygen and even at room temperature). In addition,this catalyst is stable and recyclable during oxidations. These advantages are reasonably attributed to the
The oxidation of alcohols to the corresponding aldehydes oretones is greatly important for the production of fine chemi-als [1–3]. Conventionally, these oxidations are carried out usingetallic salts as oxidants including permanganate, chromate and
romate. These processes are typically environmentally unfriendlyecause of the production of undesirable wastes and consequentlyigh costs [3b,4,5]. In recent years, environmentally benign oxi-ants such as molecular oxygen with high atom efficiency haveeen used [6]. However, heterogeneous oxidations with molecu-
ar oxygen under very mild conditions (1 atm O2 and even at roomemperature) without additives is still a great challenge [7].
It is well known that a lot of metal ions and metal nanoparti-les are catalytically active for alcohol oxidations using molecularxygen as oxidants. For example, ruthenium, copper, cobalt, andalladium have been proved to be active for this kind of reactions6d,8–11]. Recently, gold catalysts have been paid much atten-ion because of their unprecedented catalytic properties, since theorks of Hutchings and Haruta [12–15]. One of the most importantndings is that Au nanoparticles are able to catalyze the oxida-
ion of alcohols [7,16–19]. Compared with Pd and Pt catalysts,u nanoparticles on supports show superior catalytic propertiesnder mild conditions. In these cases, the activities are strongly
dependent on Au nanosizes and support types [16,17]. For exam-ple, compared with silica, CeO2 supported Au nanoparticles haveimproved activity and selectivity for the desired products in theaerobic oxidation of alcohols due to the synergetic electronic inter-actions between Au sites and CeO2 [16a]. The use of Au/Ga3Al3O9could effectively promote the conversion of alcohol oxidations,since the Ga3Al3O9 support might substantially facilitate the crucialalcohol-dehydrogenation step [16b].
More recently, we reported a communication that layered dou-ble hydroxide supported Au nanoparticles (Au/LDH) were activefor aerobic oxidation of a few alcohols at 80 ◦C [17b]. Herein, weshowed a systematical study on catalytic oxidations of a wide rangeof primary and secondary alcohols over Au/LDH catalyst under verymild conditions such as room temperature.
2. Experimental
2.1. Catalyst preparation
2.1.1. Preparation of LDHAs a typical run, 30.76 g of Mg(NO3)2·6H2O (AR, Beijing Chem
Co.) and 15 g of Al(NO3)3·9H2O (AR, Beijing Chem Co.) were dis-solved in 400 ml of water, followed by the addition of 72 g of urea
(AR, Beijing Chem Co.) under stirring at room temperature. Afterheating to boiling point, a white precipitate was formed. After boil-ing for 8 h and precipitating at room temperature for 12 h, the solidLDH was collected by filtration and washing with water.
.1.2. Preparation of Au/LDHAu/LDH was prepared by ion-exchange and NaBH4 reduction.
or a typical run, 0.22 g of hydrochloroauric acid (HAuCl4·4H2O,R, Shanghai Chem Co.) was dissolved in 80 ml of water, fol-
owed by the addition of 6 g of LDH and stirring for overnight.fter filtrating, washing and drying, the sample was transferred
o 50 ml of toluene (AR, Beijing Chem Co., dried by P2O5), fol-owed by the addition of NaBH4 (AR, Beijing Chem Co.). Aftertirring for 10 min, 15 ml of ethanol was added and the mix-ure was stirred for 6 h. Au/LDH with Au loading at 1.8% wasollected by filtration and washing with ethanol and water. Theu loading was analyzed by inductively coupled plasma (ICP)
echnique.
.1.3. Preparation of Pd/LDHFor a typical run, 0.38 g PdCl2 (AR, Shanghai Chem Co.) and NaCl
molar ratio of PdCl2/NaCl is 1:2.3) were dissolved in 80 ml of water,ollowed by the addition of 6 g of LDH and stirring for overnight. Theollowing ion-exchange and reduction procedures were the sames the preparation of Au/LDH. The Pd loading analyzed by ICP was.8%.
.1.4. Preparation of Au/TiO2, Au/MgO, Au/Fe2O3 and Au/SiO2As typical run, solid supports such as TiO2 was added to
he solution of hydrochloroauric acid. After stirring at 80 ◦C forvernight (pH = 9.0), filtrating and washing at room tempera-ure, drying at 100 ◦C for 12 h, and calcination at 400 ◦C for
h, the sample was obtained. The Au loading analyzed by ICPas 1.8%.
.2. Sample characterization
Powder X-ray diffraction patterns (XRD) were obtained with Rigaku D/MAX 2550 diffractometer with CuK� radiation� = 0.1542 nm). Transmission electron microscopy (TEM) experi-
ents were performed on a JEM-3010 electron microscope (JEOL,apan) with an acceleration voltage of 300 kV. The contents ofu and Pd were determined by ICP with a Perkin-Elmer plasma0 emission spectrometer. XPS spectra were performed a ThermoSCALAB 250 with Al K� radiation at � = 90◦ for the X-ray source,he binding energies were calibrated using the C1s peak at 284.9 eV.emperature programmed surface reaction (TPSR) of adsorbed 2-ropanol was carried out as follows: the catalysts were treatedt 300 ◦C for 3 h and cooled down to room temperature, then 2-ropanol vapor was introduced into the reaction system for 30 min.fter sweeping with Ar for 1 h, the temperature was increased
10 ◦C/min) from room temperature to 500 ◦C, and the signals of2 (M/e = 2) were recorded by mass spectrometer with a thermalonductivity detector (TCD).
.3. Catalytic tests
The oxidation of alcohols was carried out in a 50-ml glass reac-or and stirred with a magnetic stirrer. The substrate, solvent andatalyst were mixed in the reactor and heated to the reaction tem-erature. Then molecular oxygen was introduced at air pressure.fter reaction, the product was taken out from the reaction systemnd analyzed by gas chromatography (GC-14C, Shimadzu, using aame ionization detector) with a flexible quartz capillary columnoated with OV-17 and OV-1. The recyclability of Au/LDH catalyst
as carried out by separating the catalyst from the reaction sys-
em by centrifugation, washing with a large amount of methanolnd drying at 100 ◦C overnight, then the catalyst was reused in theext reaction.
Fig. 1. XRD patterns of (a) LDH support and (b) Au/LDH catalyst.
3. Results and discussion
3.1. Characterization of Au/LDH catalyst
Fig. 1 shows XRD patterns of LDH and Au/LDH samples. Theyexhibited almost the same XRD peaks, indicating that the layeredstructure of LDH is well remained after loading of Au nanoparticles.Notably, the diffraction peaks associated with Au nanoparticleswere not observed, which might be related to high dispersionof Au nanoparticles on LDH. TEM image of Au/LDH (Fig. 1(a))confirms the presence of very small Au nanoparticles distributedat 1–5 nm.
3.2. Oxidation of secondary alcohols
Table 1 presents catalytic activities and selectivities in oxidationof a typical secondary alcohol of 1-phenylethanol to acetophenonewith molecular oxygen at an atmospheric pressure over variouscatalysts. Clearly, Au/LDH catalyst was very active. After reaction at80 ◦C for 2 h in toluene, 1-phenylethanol was completely converted(Table 1, entry 1). When the temperature was decreased downto 45 ◦C, the complete conversion of 1-phenylethanol took for 4 h(Table 1, entry 2). Interestingly, when the reaction was performedat room temperature for 12 h, the conversion still had 96% (Table 1,entry 3). Compared with toluene, the use of water showed relativelylow reaction rate. When the reaction was performed at 45 ◦C for 4 h,the conversion was 40% (Table 1, entry 4). When the reaction timewas increased to 16 h, the conversion was 98% (Table 1, entry 5).In contrast, Pd/LDH and the other Au-based catalysts showed verylow conversion for this reaction. For examples, Pd/LDH was almostinactive (Table 1, entry 7). A series of Au-based catalysts withsimilar Au content with Au/LDH (Au/TiO2, Au/MgO, Au/Fe2O3 andAu/SiO2) exhibited very low conversion (11–26%, Table 1, entries9–12). Even if the presence of additional Na2CO3, Au/SiO2 still gavea low conversion (19%, Table 1, entry 13). These results indicate thatAu/LDH catalyst is very active, compared with Pd/LDH and the otherAu-based catalysts.
More importantly, when Au/LDH catalyst was recycled for 6times, the conversion still have 97% (Table 1, entry 6), which isstill comparable with the fresh catalyst (loss of activity less than3%, Table 1, entry 1). These results indicate that Au/LDH catalyst
was stable and reusable in aerobic oxidation of 1-phenylethanol.Sample TEM images (Fig. 2) showed that Au nanoparticle sizesof the recycled catalyst were similar to those of the fresh cata-lyst, indicating that Au nanoparticles are basically stable during
406 L. Wang et al. / Catalysis Today 175 (2011) 404– 410
Table 1Aerobic oxidation of 1-phenylethanol over various Au catalysts.
.
Entry Catalyst Solvent Temperature (◦C) Time (h) Conversion (%) Selectivity (%)
Reaction conditions: 0.6 mmol of 1-phenylethanol, 80 mg of catalyst, 10 ml of solvent, O2 with rate 30 ml/min, room temperature at 26 ◦C.a Recycled for 6 times.b Addition of t-butyl hydroperoxide (TBHP, 5 mol% based on substrate) as initiator.c Formation of benzaldehyde by-product.d Addition of 1 mmol of Na2CO3 base.
Fig. 2. TEM images and Au size distribution of (a) Au/LDH catalyst and (b) Au/LDH catalyst recycled for 6 times.
L. Wang et al. / Catalysis Toda
3.02.52.01.51.00.50.0
0
10
20
30
40
50
60
70C
onve
rsio
n (%
)
Time (h)
Removal of Au/LDH cata lyst
Fig. 3. Dependence of catalytic activity in oxidation of 1-phenylethanol on time for areaction system before and after separation of Au/LDH catalyst. Reaction conditions:0w
ctc
c(1or
rBwr
TA
R
.6 mmol of 1-phenylethanol, 80 mg of Au/LDH catalyst, 10 ml of toluene, 80 ◦C, O2
ith rate 30 ml/min.
atalyst recycles. The excellent stability of Au/LDH catalyst is poten-ially important for its industrial applications for production of finehemicals.
It is worth noting that aerobic oxidation of 1-phenylethanolould occur in the absence of solvent. When a reaction system160 mmol and 10 mg of Au/LDH) was performed for 0.5 h at60 ◦C, Au/LDH gives a extremely high turnover frequency (TOF)f 21,040 h−1, which is comparable with the most active Au specieseported in literature [16b,23].
Fig. 3 shows dependence of catalytic conversion on time for a
eaction system before and after separation of Au/LDH catalyst.efore the removal of Au/LDH catalyst, the conversion increasedith time. However, after separation of Au/LDH catalyst from the
eaction system, the conversion became a constant. These results
able 2erobic oxidation of secondary alcohols over Au/LDH catalyst.
Entry Substrate Product Temperature (◦
1 R.T.
2 R.T.
3 45
4 80
5 R.T.
6 45
7 80
8 80
eaction conditions: 0.6 mmol of alcohol, 80 mg of catalyst, 10 ml of solvent, O2 with rate
y 175 (2011) 404– 410 407
indicate that there is no leaching of active sites from Au/LDHcatalyst during heterogeneous oxidation of 1-phenylethanol to ace-tophenone with molecular oxygen.
Table 2 presents aerobic oxidation of various secondaryalcohols over Au/LDH catalyst. The conversion of p-methyl-�-phenylethanol and p-methoxy-�-phenylethanol was very high,giving at 95% and 99%, respectively (Table 2, entries 1 and 2).Diphenylmethanol also had good conversion (92%) (Table 2, entry3), but 1-(2-aminophenyl)ethanol showed relatively low conver-sion (81%, Table 2, entry 4), which might be related to the presenceof amine groups in 1-(2-aminophenyl)ethanol. Compared withthese aromatic secondary alcohols, aliphatic secondary alcoholsexhibit low activities (Table 2, entries 5–8). For example, cyclo-hexanol had conversion at 60% at room temperature (Table 2,entry 5), and 2-cyclohexan-1-ol gave conversion at 74%. Normally,the oxidation of olefinic alcohols is considered as a model reac-tion because side reactions such as hydrogenation, hydrogenolysis,and decarbonylation exist during the oxidative process. Interest-ingly, a typical olefinic alcohol of 2-cyclohexan-1-ol exhibited veryhigh selectivity for 2-cyclohexan-1-one (>99.5%, Table 2, entry 6),suggesting that the side reactions are free in the oxidation. Forstraight-chain alcohols, 2-hexanol and 2-butanol still had conver-sion of 43 and 33% at 80 ◦C (Table 2, entries 7 and 8). These resultsindicate that Au/LDH catalyst is very active for aerobic oxidation ofa wide range of secondary alcohols.
3.3. Oxidation of primary alcohols
Generally, compared with secondary alcohols, the oxidationof primary alcohols shows relatively low activities [16]. Table 3presents catalytic activities and selectivities in aerobic oxidationof benzyl alcohol over Au/LDH catalyst, and the choice of benzylalcohol as a model reaction is due to the presence of side reactions
such as over-oxidation of benzaldehyde and esterification of benzylalcohol with benzoic acid. Notably, Au/LDH catalyst was very activeand selective for the formation of benzaldehyde in the presence oftoluene solvent (Table 3, entries 1–3). When the temperature was
C) Solvent Time (h) Conversion (%) Selectivity (%)
Toluene 20 95 >99.5
Toluene 20 99 >99.5
Toluene 8 92 >99.5
Toluene 8 81 92
Toluene 24 60 >99.5
Toluene 12 74 >99.5
Toluene 24 43 >99.5
Toluene 40 33 >99.5
30 ml/min, room temperature at 26 ◦C.
408 L. Wang et al. / Catalysis Today 175 (2011) 404– 410
Table 3Aerobic oxidation of benzyl alcohol over various Au catalysts.
.
Entry Catalyst Solvent Temperature (◦C) Time (h) Conversion (%) Selectivitya (%)
Reaction conditions: 0.6 mmol of benzyl alcohol, 80 mg of catalyst, 10 ml of solvent, O2 with rate 30 ml/min.a Selectivity for benzaldehyde.b Addition of t-butyl hydroperoxide (TBHP, 5 mol% based on substrate) as initiator.
8(tirbcl
t(twttduwpcaacb
TA
R
c Addition of 1 mmol of Na2CO3 base.
0 ◦C, the catalyst gave the conversion at 89% and selectivity at 93%Table 3, entry 1); when the temperature was reduced to 45 ◦C,he catalyst showed lower conversion (77%) but higher selectiv-ty (96%) (Table 3, entry 2); when the reaction was performed atoom temperature, the conversion was much lower (49%) and theenzaldehyde was selectively formed (>99%) (Table 3, entry 3). Inontrast, Pd/LDH and the other Au-based catalysts exhibited veryow conversion (1–25%, Table 3, entries 7–13).
Interestingly, when water was used as solvent in the oxida-ion, Au/LDH catalyst showed high conversion and low selectivityTable 3, entries 4–6), compared with toluene solvent. For example,he conversion of benzyl alcohol at 45 ◦C in water can reach 90%,hich is much higher than that in toluene solvent (77%). On the con-
rary, the selectivity in water was only 71%, which is less than that inoluene solvent (96%). This phenomenon is possibly assigned to theifference in solubility of alcohols [6a,20] and in reactivity of prod-cts [19b]. For example, primary alcohols have better solubility inater than secondary alcohols, resulting in higher conversion ofrimary alcohols than secondary alcohols [6a,20]. However, waterould react with aldehyde oxidized by primary alcohols to form
ldehyde hydrate, which could be easily oxidized into carboxyliccid, a major by-product of primary alcohol oxidations [19b]. Inontrast, the acetones oxidized by secondary alcohols in water areasically stable.
able 4erobic oxidation of primary alcohols over Au/LDH catalyst.
Entry Substrate Product Temperature (◦C)
1 45
2a 80
eaction conditions: 0.6 mmol of alcohol, 80 mg of catalyst, 10 ml of solvent, O2 with ratea O2 pressure at 0.5 MP.
Table 4 presents catalytic activities and selectivities in oxida-tion of 2-phenylethanol and 1-hexanol in the presence of water.2-phenylethanol had good conversion (87%) and selectivity forphenylacetaldehyde (81%), while 1-hexanol showed low activity(14%) and selectivity for n-hexaldehyde (22%).
3.4. XPS spectra of Au/LDH, Au/TiO2, and Au/SiO2 catalysts
To understand the highly active Au nanoparticles on the sur-face of LDH support, XPS spectra of Au/LDH, Au/TiO2, and Au/SiO2catalysts have been compared, as shown in Fig. 4. Au/LDH cata-lyst showed Au 4f7/2 binding energy at 83.2 eV (Fig. 4(a)), whichis obviously lower than that of Au/TiO2 (83.5 eV, Fig. 4(b)) and ofAu/SiO2 catalyst (84.0 eV, Fig. 4(c)). Compared with Au/TiO2 andAu/SiO2, the downshift of Au 4f7/2 over Au/LDH indicates that Aunanoparticles are negative charged, which might be resulted fromthe strong interaction between Au nanoparticles with LDH support[17a,21,22]. The negatively charged Au nanoparticles are favorablefor activation of molecular oxygen in aerobic oxidation of alcohols[23]. It was reported that a hydrogenation step is key step for the
oxidation of alcohols [19b,24,25]. In addition, the support alkalin-ity is important to facilitate the dehydrogenation step [19b,24]. Thisconclusion is also confirmed by the fact that the addition of solidbase (Na2CO3) could effectively enhance catalytic activities in this
Solvent Time (h) Conversion (%) Selectivity (%)
H2O 24 87 81
H2O 24 14 22
30 ml/min.
L. Wang et al. / Catalysis Today 175 (2011) 404– 410 409
95908580
Binding Energy (eV)
Inte
nsity
(a.u
.)
83.2 eV
a
95908580
Binding Energy (eV)
Inte
nsity
(a. u
.)
83.5 eV b
95908580
Inte
nsity
(a. u
.)
84.0 eV c
wtirAbotLiga
400300200100
(d)
(c)
(b)H2 s
igna
l int
ensi
ty (a
.u.)
Temperature oC
(a)
[
Binding Energy (eV)
Fig. 4. Au4f XPS spectra of (a) Au/LDH, (b) Au/TiO2, and (c) Au/SiO2 catalysts.
ork (Table 1, entries 11 and 12; Table 2, entries 12 and 13). Hence,he dehydrogenation abilities of various samples were systemicallynvestigated. Fig. 5 shows the temperature programmed surfaceeaction (TPSR) curves of 2-propanol on Au/TiO2, Au/SiO2, LDH, andu/LDH samples for H2 mass intensity. Notably, no H2 signals coulde detected for Au/TiO2 and Au/SiO2 samples. However, two obvi-us peaks assigned to H2 signals appeared at 294 and 420 ◦C, whenhe TPSR processes were carried out by adsorption 2-propanol on
DH sample. Interestingly, Au/LDH gives a H2 peak with strongerntensity at much lower temperature of 270 ◦C. These results sug-est that LDH support has excellent dehydrogenation abilities fordsorbed 2-propanol, and the presence of Au nanoparticles on LDH
[
[
Fig. 5. TPSR spectra of 2-propanol adsorbed on (a) Au/TiO2, (b) Au/SiO2, (c) LDH and(d) Au/LDH for H2 signals.
can greatly facilitate this process, which could be the reason of highactivities of Au/LDH for the alcohol oxidations.
4. Conclusions
Extremely small Au nanoparticles distributed at 1–5 nm on LDHsupport (Au/LDH) is successfully prepared by ion-exchange andreduction procedures. Compared with the other Au-based cat-alysts, Au/LDH catalyst shows good activities, high selectivities,and recyclability in aerobic oxidation of a wide range of pri-mary and secondary alcohols under very mild conditions, whichis of great importance for potentially industrial production of finechemicals.
Acknowledgements
This work is supported by the National Natural Science Founda-tion of China (20973079) and State Basic Research Project of China(2009CB623501).
References
[1] R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds,Academic Press, New York, 1981.
[3] (a) G.A. Barf, R.A. Sheldon, J. Mol. Catal. A: Chem. 102 (1995) 23;(b) B. Notari, Stud. Surf. Sci. Catal. 60 (1991) 343.
[4] F.A. Luzzio, Organic Reactions, 53, Wiley, New York, 1998, p. 1.[5] H. Yang, B. Li, Synth. Commun. 21 (1991) 1521.[6] (a) C.Y. Ma, B.J. Dou, J.J. Li, J. Cheng, Q. Hu, Z.P. Hao, S.Z. Qiao, Appl. Catal. B:
Environ. 92 (2009) 202;(b) G.J. ten Brink, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636;(c) T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037;(d) B.M. Trost, Science 254 (1991) 1471.
[7] T. Wang, H. Shou, Y. Kou, H. Liu, Green Chem. 11 (2009) 562.[8] T. Inokuchi, K. Nakagawa, S. Torii, Tetrahedron Lett. 36 (1995) 3223.[9] (a) A. Dijksman, A.M.I. Payeras, I.W.C.E. Arends, R.A. Sheldon, J. Am. Chem. Soc.
123 (2001) 6826;(b) I.E. Marko, P.R. Giles, M. Tsukazaki, S.M. Brown, C.J. Urch, Science 274 (1996)2044.
10] (a) J. Chen, Q.H. Zhang, Y. Wang, H.L. Wan, Adv. Synth. Catal. 350 (2008) 453;(b) F. Li, Q.H. Zhang, Y. Wang, Appl. Catal. A: Gen. 334 (2008) 217;(c) R. Tang, S.E. Diamond, N. Nery, F. Mares, J. Chem. Soc. Chem. Commun. (1978)562.
11] (a) X. Meng, K. Lin, X. Yang, Z. Sun, D. Jiang, F.-S. Xiao, J. Catal. 218 (2003) 460;(b) X. Meng, Z. Sun, R. Wang, S. Lin, J. Sun, M. Yang, K. Lin, D. Jiang, F.-S. Xiao,Catal. Lett. 76 (2001) 105.
12] (a) G.J. Hutchings, J. Catal. 96 (1985) 292;(b) M. Haruta, T. Koboyashi, H. Sano, N. Yamada, Catal. Lett. 16 (1987) 405.
14] (a) A. Corma, H. Garcia, Chem. Soc. Rev. 37 (2008) 2096;(b) A. Grirrane, A. Corma, H. Garcia, J. Catal. 268 (2009) 350;(c) W. Yan, S. Brown, Z. Pan, S.M. Mahurin, S.H. Overbury, S. Dai, Angew. Chem.Int. Ed. 45 (2006) 3614;(d) W. Yan, S.M. Mahurin, S.H. Overbury, S. Dai, Top. Catal. 39 (2006) 199.
15] (a) X. Zhang, H. Shi, B. Xu, Angew. Chem. Int. Ed. 44 (2005) 7132;(b) D. He, H. Shi, Y. Wu, B. Xu, Green Chem. 9 (2007) 849;(c) H. Sun, F.-Z. Su, J. Ni, Y. Cao, H.-Y. He, K.-N. Fan, Angew. Chem. Int. Ed. 48
(2009) 4390.
16] (a) A. Abad, P. Concepcion, A. Corma, H. Garcia, Angew. Chem. Int. Ed. 44 (2005)4066;(b) F.-Z. Su, Y.-M. Liu, L.-C. Wang, Y. Cao, H.-Y. He, K.-N. Fan, Angew. Chem. Int.Ed. 47 (2008) 334.
[
[[
y 175 (2011) 404– 410
17] (a) L. Wang, X. Meng, B. Wang, W. Chi, F.-S. Xiao, Chem. Commun. 46 (2010)5003;(b) L. Wang, X. Meng, F.-S. Xiao, Chin. J. Catal. 31 (2010) 943.
18] H. Tsunoyama, H. Sakurai, Y. Negishi, T. Tsukuda, J. Am. Chem. Soc. 127 (2005)9374.
19] (a) S. Biella, L. Prati, M. Rossi, J. Catal. 206 (2002) 242;(b) S. Carretin, P. McMorn, P. Johnston, K. Griffin, G.J. Hutchings, Chem. Com-mun. (2002) 696.
20] N. Dimitratos, A. Villa, D. Wang, F. Porta, D.S. Su, L. Prati, J. Catal. 244 (2006)113.
21] Y.F. Han, Z.Y. Zhong, K. Ramesh, F.X. Chen, L.W. Chen, J. Phys. Chem. C 111 (2007)3163.
22] F.-Z. Su, M. Chen, L.-C. Wang, X.-S. Huang, Y.-M. Liu, Y. Cao, H.-Y. He, K.-N. Fan,
23] H. Tsunoyama, N. Ichikuni, H. Sakurai, T. Tsukuda, J. Am. Chem. Soc. 131 (2009)7086.
24] P. Haider, A. Baiker, J. Catal. 248 (2007) 175.25] C. Keresszegi, T. Mallat, J.D. Gruwaldt, A. Baiker, J. Catal. 225 (2004) 138.
